Changes in the optical length of a laser resonator change the frequency of oscillation of the laser, which must be kept stable for coherent laser emission. Conventional single-mode lasers are subject to short-term frequency fluctuation caused by very small optical path length variations associated with plasma density fluctuations. Long-term frequency variations are caused by thermally-induced optical path length change and mechanically-induced vibration. A one-part-per-million change in the optical path length will produce a 28 MHz frequency shift in a CO.sub.2 laser, for example. Hence, optical path length must be held to a few-tenths of a micrometer long-term and one thousand times less for short time intervals associated with the duration of a single transmission.
An electronically scanned laser (ESL) is subject to the above-mentioned problems and is further complicated by the effects induced by scanning the directional mode within the resonator optics. An optical path difference exists for mode direction shifts due to tiny imperfections in the optical elements. The resulting frequency shift can be considered long-term provided the beam position is constant during a measurement interval. Transient path length changes thermally induced by the intra-cavity laser beam will produce a short-term frequency "chirp" which is preferably kept exceedingly small.
The frequency at which a laser oscillates is determined broadly by the laser fluorescent line width and narrowly by the resonance of the optical cavity. Attempts to solve the problem of frequency fluctuation or to minimize its effect have ranged from heroic measures to control the physical length of the resonator to complex feedback systems.
Additionally, a phase conjugate reflector may be used at one end of the resonator to ensure that the reflective wave will match the phase of the wavefront leaving the opposite reflector; that is, the optical cavity will provide a high "Q" feedback at any frequency and the laser fluorescent line will be the sole determinant of the frequency of oscillation. The product of laser gain and reflectivity of the conjugate reflector must, however, be sufficient to maintain oscillation. Known schemes for meeting this requirement, such as high peak power four wave mixing, are generally complicated and inefficient.